1. Field of the Invention
This invention is directed to a direct-to-press imaging method and system useful in lithographic printing. More particularly, the imaging method and system of this invention permits an imageable coating to be reapplied to a printing cylinder already having an imaged coating residing thereon, without the need for substantially removing the prior imaged coating before reapplying the new imageable coating.
2. Background Information
The art of lithographic printing is based upon the immiscibility of oil and water, wherein the oily material or ink is preferentially retained by the image area and the water or fountain solution is preferentially retained by the non-image area. When a suitably prepared surface is moistened with water and an ink is then applied, the background or non-image area retains the water and repels the ink while the image area accepts the ink and repels the water. The ink on the image area is then transferred to the surface of a material upon which the image is to be reproduced, such as paper, cloth and the like. Commonly the ink is transferred to an intermediate material called the blanket which in turn transfers the ink to the surface of the material upon which the image is to be reproduced.
A very widely used type of lithographic printing plate has a light-sensitive coating applied to an aluminum base support. The coating may respond to light by having the portion which is exposed become soluble so that it is removed in the developing process. Such a plate is referred to as positive-working. Conversely, when that portion of the coating which is exposed becomes hardened, the plate is referred to as negative-working. In both instances the image area remaining is ink-receptive or oleophilic and the non-image area or background is water-receptive or hydrophilic. The differentiation between image and non-image areas is made in the exposure process where a film is applied to the plate with a vacuum to insure good contact. The plate is then exposed to a light source, a portion of which is composed of UV radiation. In the instance where a positive plate is used, the area on the film that corresponds to the image on the plate is opaque so that no light will strike the plate, whereas the area on the film that corresponds to the non-image area is clear and permits the transmission of light to the coating which then becomes more soluble and is removed. In the case of a negative plate the converse is true. The area on the film corresponding to the image area is clear while the non-image area is opaque. The coating under the clear area of film is hardened by the action of light while the area not struck by light is removed. The light-hardened surface of a negative plate is therefore oleophilic and will accept ink while the non-image area which has had the coating removed through the action of a developer is desensitized and is therefore hydrophilic.
Lithographic plates may be divided into classes based upon their affinity for printing ink. Those which require dampening water which is fed to the non-image areas of the plate, forms a water film and acts as an ink-repellant layer; this is the so-called fount solution. Those which require no fount solution are called driographs or water-less lithographic plates. Most lithographic plates at present in use are of the first type and require a fount-solution during printing.
Image forming by digital computer aided design of graphical material or text is well known. Electronically derived images of words or graphics presented on the CRT of a digital computer system can be edited and converted to final hard copy by direct printing with impact printers, laser printers or ink jet printers. This manner of printing or producing hard copy is extremely flexible and useful when print runs of no more than a few thousand are required but the print process is not feasible for large runs measured in the tens or hundreds of thousands of pieces. For large runs, printing by lithographic plate is still the preferred process with such plates prepared by the process of photographic image transfer.
As disclosed, for example, at col. 2, line 21 to col. 3, line 10 of co-assigned U.S. Pat. No. 5,908,705 and the references cited therein, and U.S. Pat. No. 5,339,737 and the references cited therein, lasers and their amenability to digital control have stimulated a substantial effort in the development of laser-based imaging systems. Early examples utilized lasers to etch away material from a plate blank to form an intaglio or letterpress pattern. This approach was later extended to production of lithographic plates, e.g., by removal of a hydrophilic surface to reveal oleophilic underlayers. These systems generally require high-power lasers which are expensive and slow.
A second approach to laser imaging involves the use of thermal-transfer materials. With these systems, a polymer sheet transparent to the radiation emitted by the laser is coated with a transferable material. During operation the transfer side of this construction is brought into contact with an acceptor sheet, and the transfer material is selectively irradiated through the transparent layer. Irradiation causes the transfer material to adhere preferentially to the acceptor sheet. The transfer and acceptor materials exhibit different affinities for fountain solution and/or ink, so that removal of the transparent layer together with non-irradiated transfer material leaves a suitably imaged, finished plate. Typically, the transfer material is oleophilic and the acceptor material hydrophilic. Plates produced with transfer-type systems tend to exhibit short useful lifetimes due to the limited amount of material that can effectively be transferred. In addition, because the transfer process involves melting and resolidification of material, image quality tends to be visibly poorer than that obtainable with other methods.
Lasers have also been used to expose a photosensitive blank for traditional chemical processing. In an alternative to this approach, a laser has been employed to selectively remove, in an imagewise pattern, an opaque coating that overlies a photosensitive plate blank. The plate is then exposed to a source of radiation with the unremoved material acting as a mask that prevents radiation from reaching underlying portions of the plate. Either of these imaging techniques requires the cumbersome chemical processing associated with traditional, non-digital platemaking.
Lithographic printing plates suitable for digitally controlled imaging by means of laser devices have also been disclosed in the prior art. Here, laser output ablates one or more plate layers, resulting in an imagewise pattern of features on the plate. Laser output passes through at least one discreet layer and imagewise ablates one or more underlying layer. The image features produced exhibit an affinity for ink or an ink-adhesive fluid that differs from that of unexposed areas. The ablatable material used to describe the image is deposited as an intractable, infusible, IR absorptive conductive polymer under an IR transparent polymer film. As a consequence, the process of preparing the plate is complicated and the image produced by the ablated polymer on the plate does not yield sharp and distinct printed copy.
Because it is desirable to avoid the use of a developer, so-called xe2x80x9cprocesslessxe2x80x9d plates have also been developed. Processless plates are imaged prior to being mounted on a printing press. The imaged plate is then mounted on the press, and the press is run briefly to permit the non-imaged areas of the plate to be washed off by the fount solution.
However, as discussed, for example in U.S. Pat. No. 5,713,287, operations involving xe2x80x9coff-pressxe2x80x9d imaging as employed in processless plate technology and subsequent manual mounting of the plate on the press are relatively slow and cumbersome. Accordingly, xe2x80x9con pressxe2x80x9d imaging methods have been developed to generate the desired image directly on a plate (on the press) or directly on a printing cylinder. For example:
U.S. Pat. No. 5,317,970 is directed to a method for reversibly regenerating a printing form such as a printing form cylinder. More particularly, after the printing form is imaged, an ionized reactive gas is conducted to the surface of the printing form, and applied thereto, thereby reacting with hydrophobic particles on the surface of the printing form and removing these particles, thus enabling the image on the printing form to be erased so that the form may be reimaged and reused;
U.S. Pat. No. 5,992,323 is directed to a printing process which employs an intermediate transfer element formed in this press by depositing and fixing a hardenable material onto a substrate which is not dismantleable from the press. The substrate and hardenable material each have a different affinity for a colorant vehicle employed in printing, thus the intermediate transfer element includes zones having an affinity for the colorant vehicle and zones without such affinity. After a printing phase, the intermediate transfer element is dismantled by removing the hardenable material by, for example, melting the hardenable material, and removing the hardenable material to permit putting a new hardenable material into place on the substrate;
U.S. Pat. No. 5,713,287 is directed to a system in which a printing cylinder is spray coated with a polymer, and the polymer surface is modified by selective laser irradiation to change its affinity to printing ink. As discussed at col. 5, lines 47-65, after printing, the cylinder is cleaned on the press using a cleaning station to remove ink and the imaged polymer, although complete cleaning is not required. A new polymer coat is then applied over the residue of the prior imaged polymer coating, and subsequently imaged; and
U.S. Pat. No. 5,996,499 is directed to a method of on-site preparation of a lithographic printing surface such as a printing cylinder in which a coating is applied to the printing surface, and the surface is imagewise exposed using IR radiation. The coating is a combination of a first thermally reactive chemical which, after imaging, changes its affinity to either ink, water or both, and a second chemical which increases the IR sensitivity of the first chemical after mixed therewith. As discussed at col. 4, lines 27-30, cleaning of the printing surface is performed after each print run, prior to recoating.
In view of the foregoing, it would be advantageous to employ a processless xe2x80x9cdirect-to-pressxe2x80x9d imaging method and system which does not require substantially removing a previous imaged composition residing on a printing cylinder prior to recoating and reimaging of the surface. It is one object of this invention to provide such an imaging method and system. Other objects, features and advantages of this invention will be readily apparent to those skilled in the art.
A direct-to-press imaging method comprises:
(m) applying an imageable coating to a printing cylinder, wherein the imageable coating comprises a composition which changes affinity for a printing fluid (i.e. fount solution and/or ink) upon exposure to imaging radiation such as radiation delivered imagewise via a laser, and the imageable coating is substantially insoluble in the printing fluid;
(n) imagewise exposing the imageable coating to imaging radiation to obtain an imaged coating;
(o) printing a plurality of copies of an image from the imaged coating; and
(p) reapplying the imageable coating as desired by repeating steps (a) through (c) at least once without substantially removing the prior imaged coating before reapplying the imageable coating.
The composition which changes affinity for the printing fluid (i.e. a printing ink, fount solution or combination thereof) upon exposure to imaging radiation is preferably a thermally switchable polymer. The imageable coating is preferably applied by spraying upon the preexisting imaged coating which has previously been contacted with a printing fluid and used to deliver a printed image. Although remaining printing fluid must be substantially removed from the imaged coating prior to application of the subsequent imageable coating, application of the subsequent imageable coating is achieved without substantially removing the prior imaged coating itself.
The system of this invention comprises:
(m) a printing cylinder capable of receiving an imageable coating;
(n) a coating unit mounted proximate to the printing cylinder;
(o) a thin layer of an imageable composition formed on the printing cylinder by the coating unit, wherein the imageable coating comprises a composition which changes affinity for a printing fluid upon exposure to imaging radiation, preferably a thermally switchable polymer, and the imageable coating is substantially insoluble in the printing fluid;
(p) an imaging unit mounted proximate to the printing cylinder and operable to imagewise expose the imageable coating to imaging radiation to obtain an imaged coating;
(q) a printing fluid application unit mounted proximate to the printing cylinder and configured to apply printing fluid to the imaged coating to form a printing fluid image thereon; and
(r) a transfer system mounted proximate to the printing cylinder and configured to transfer the printing fluid image to a print-receiving medium; and
(s) a removal system for substantially removing printing ink, fount solution, water or a combination thereof from the imaged coating after transfer of the printing fluid image to a print receiving medium without substantially removing the imaged coating.
Removal of the printing ink, water, fount solution or a combination thereof may be achieved using, for example, a conventional blanket washer, or by running the press for a small number of additional impressions, without feeding ink or fountain solution, upon completion of printing using a prior imaged coating to transfer the residual printing ink, fount solution, water or combination thereof from the prior imaged coating onto the paper. In one embodiment, this may be achieved via a two step process by first turning off the ink supply and thereafter turning off the fount solution supply.